THE BELGIAN GRAND PRIX this weekend is “very difficult” to predict, according to Mercedes chief Toto Wolff.

Lewis Hamilton returns to the asphalt looking to build on back-to-back wins before the month-long summer break in Germany and Hungary.

Those successes gave him a 24-point advantage over Ferrari rival Sebastian Vettel with nine races of the season remaining - starting with Spa this Sunday (2.10pm).

But while the Silver Arrows lead both championships, their lead is not reflective of an on-track dominance with Ferrari having pushed them all the way so far this season.

The advantage has switched between the two teams throughout the year so far but the Scuderia have generally boasted the faster package, with Hamilton acknowledging the challenge that poses to Mercedes - who have snapped up both titles in all four years of the turbo hybrid engine era so far.

Hamilton said last month: “This year Ferrari really do have the upper hand pace-wise. But I think all around, as a whole, we have hopefully done a slightly better job.

“We’ve got work to do; we’ve got things to improve. We’ve got to try and catch them but we’ve got to continue to keep rising with all the other elements, which allows us to beat Ferrari when they don’t bring their A-game.”

The 33-year-old is looking for a fourth triumph at the fast-paced Spa-Francorchamps circuit to join compatriot Jim Clark as the joint-most successful British driver in the Belgian Grand Prix’s history this weekend.

But Mercedes executive director Wolff has warned that the competition will be close between themselves, Ferrari and Red Bull.

"It's very difficult to make any predictions for Spa," Wolff told Mercedes’ official website.

"If this season has taught us one thing, it's that there are no clear favourites for specific tracks anymore and that it's not always the fastest car that wins.